fix: sed line number incorrect

[iNecas]

The original code for prefixing line numbers was not working, as it was adding less extra space than needed:

seq 3 | sed = | sed 'N; s/^/   /; s/ *\(.\{6,\}\)\n/\1/'
   1
1
   2
2
   3
3

Adding few more spaces fixes the issue:

seq 3 | sed = | sed 'N; s/^/     /; s/ *\(.\{6\}\)\n/\1 /'
     1 1
     2 2
     3 3

I've also changed \{6,\} to \{6\}, as the extra , shouln't be needed and might be confusing.